There are many applications which can use materials that can effectively absorb hydrogen and then release hydrogen. These materials can be used to absorb hydrogen in confined spaces and then upon release of the hydrogen from the materials, they can be used again to absorb hydrogen. Applications where hydrogen absorbers can be used are fuel cells, devices for the separation of hydrogen from various gases; isotope separators; heat pumps and refrigerators; heat engines; hydrogen storage devices; temperature sensors and actuators and permanent magnet products. In addition to these applications, good hydrogen storage compounds that can absorb and then release hydrogen can be used in electrochemical cells, such as secondary electrochemical cells. In secondary or rechargeable cells, a negative electrode can be used that is capable of reversibly electrochemically storing hydrogen. These cells generally use a positive electrode made of a nickel hydroxide material along with an alkaline electrolyte. During charging of these cells, the negative electrode material is charged by the electrochemical absorption of hydrogen and the electrochemical evolution of a hydroxyl ion. Upon discharging, the stored hydrogen is released to form a water molecule and evolve an electron which is subsequently used in the charging cycle of the cell. In conventional rechargeable nickel metal hydride cells, the positive electrode upon charging releases water and evolves an electron which is used during discharge to form a hydroxyl ion. Thus a hydrogen storage compound can be used effectively in a rechargeable cell, such as a nickel metal hydride cell.
The prior art on hydrogen storage materials is discussed in detail in U.S. Pat. No. 5,096,667. The following is a recitation of the prior art on hydrogen storage material that is recited in U.S. Pat. No. 5,096,667. One type of a hydrogen storage material is AB.sub.2 type hydrogen storage alloys. Early reported members of the AB.sub.2 class of hydrogen absorbing material were the binaries ZrCr.sub.2, ZrV.sub.2, and ZrMo.sub.2. These were reported to be thermal hydrogen storage alloys by A. Pebler et al. in "Transactions of the Metallurgical Society", 239, 1593-1600 (1967). Another early member of this class is the Mg-Ni thermal hydrogen storage alloy described by J. J. Reilly et al. in "The Reaction of Hydrogen With Alloys of Magnesium and Nickel and the Formation of Mg.sub.2 NiH.sub.4 " Inorganic Chem. (1968) 7, 2254. F. H. M. Spit, et al. describe a class of ZrNi binary thermal hydrogen storage alloys in "Hydrogen Sorption By The Metallic Glass Ni.sub.64 Zr.sub.36 And By Related Crystalline Compounds," Scripta Metallurgical 14, (1980) 1071-1076. This reference describes the thermodynamics of gas phase hydrogen adsorption and desorption in the ZrNi.sub.2 binary system. Subsequently, an article by F. H. M. Spit, et al., "Hydrogen Sorption in Amorphous Ni(Zr,T.sub.1) Alloys", Zeitschrift Fur Physikaisch Chemie Neue Folge Bd., 225-232 (1979), reports the gas phase hydrogen sorption and desorption kinetics of thermal hydrogen storage processes in Zr.sub.36.3 Ni.sub.63.7 and Ti.sub.29 Zr.sub.9 Ni.sub.62. Zirconium-manganese binary thermal hydrogen storage alloys were disclosed, for example, by F. Pourarian et al., in "Stability and Magnetism of Hydrides of Nonstoichiometric ZrMn.sub.2 ", J. Phys. Chem. 85, 3105-3111.
Manganese-nickel binary thermal hydrogen storage alloys were described for thermal hydrogen storage in automotive applications by H. Buchner in "Perspectives For Metal Hydride Technology", Prog. Energy Combust. Sci. 6, 331-346. Ternary zirconium, nickel, manganese thermal hydrogen storage alloys are described in, for example, an article by A. Seasick et al., "Thermodynamic Properties of Zr(Ni.sub.x Mn.sub.1-x).sub.2 --H.sub.2 Systems," Mat. Res. Bull. 19 1559-1571 (1984).
Six component thermal hydrogen storage alloys of the general AB.sub.2 type are described in German Patentschrift DE 31-51-712-CI for Titanium Based Hydrogen Storage Alloy With Iron And/or Aluminum Replacing Vanadium and Optionally Nickel. A further teaching relating to multi-component thermal hydrogen storage alloys of this general type is disclosed in German Patentschrift DE 30-23-770-C2 for Titanium Manganese Vanadium Based Laves Phase Material With Hexagonal Structure, Used As Hydrogen Storage Material. The key teaching of this reference is that the nickel in a six component Ti--Zr--Mn--Cr--V--Ni alloy can be partially replaced by Co and/or Cu to give a lower cost thermal hydrogen storage alloy.
Other Laves phase materials are disclosed in U.S. Pat. 4,160,014 for Hydrogen Storage Material. Specifically, an AB.sub.a type thermal hydrogen storage material is disclosed where A is at least 50 atomic percent Ti, balance one or more of Zr or Hf; B is at least 30 atomic percent Mn, balance one or more of Cr, V, Nb, Ta, Mo, Fe, Co, Ni, Cu, and rare earths; and a is from 1.0 to 3.0.
All of the AB.sub.2 hydrogen storage alloys described are thermal hydrogen storage alloys. Prior art Laves phase-type electrochemical hydrogen storage alloys are shown, for example, in Laid Open European Patent Application 0-293 660.
Another suitable class of electrochemical hydrogen storage alloys are the Ti--V--Zr--Ni type active materials for use as a negative electrode. These materials are disclosed in U.S. Pat. No. 4,551,400. These materials reversibly form hydrides in order to store hydrogen and have the generic Ti--V--Zr--Ni composition, where at least Ti, V, and Ni are present with at least one or more of Cr, Zr, and Al.
Other Ti--V--Zr--Ni materials may also be used for the rechargeable hydrogen storage negative electrode. One such family of materials are those described in U.S. Pat. No. 4,728,586 for ENHANCED CHARGE RETENTION ELECTROCHEMICAL HYDROGEN STORAGE ALLOYS AND AN ENHANCED CHARGE RETENTION ELECTROCHEMICAL CELL. This patent describes a specific sub-class of the Ti--V--Ni--Zr hydrogen storage alloys comprising titanium, vanadium, zirconium, nickel, and a fifth component, chromium.
An alternative class of hydrogen storage alloys are the AB.sub.5 type hydrogen storage alloys. These alloys differ in chemistry, microstructure, and electrochemistry from the AB.sub.2 and the V--Ti--Zr--Ni--Cr types of electrochemical hydrogen storage alloys. Rechargeable batteries utilizing AB.sub.5 type negative electrodes are described, for example, in U.S. Pat. Nos. 3,874,928, 4,214,043, 4,107,395, 4,107,405, 4,112,199, 4,125,688, 4,214,043, 4,216,274, 4,487,817, 4,605,603, 4,621,034, 4,696,873 and 4,699,856.
U.S. Pat. No. 5,096,667 discloses a reversible, electrochemical cell having a high electrochemical activity, hydrogen storage negative electrode. The negative electrode is formed of a reversible, multicomponent, multiphase, electrochemical hydrogen storage alloy. The hydrogen storage alloy is capable of electrochemically charging and discharging hydrogen in alkaline aqueous media. In one preferred exemplification, the hydrogen storage alloy is a member of the family of hydrogen storage alloys derived from the V--Ti--Zr--Ni and V--Ti--Zr--Ni--Cr alloys in which the V, Ti, Zr, Ni and Cr are partially replaced by one or more modifiers, and the alloy has the composition: EQU (V.sub.y'-y Ni.sub.y Ti.sub.x'-x Zr.sub.x Cr.sub.z).sub.a M.sub.b 'M.sub.c "M.sub.d '"M.sub.e.sup.IV
where x' is between 1.8 and 2.2; x is between 0 and 1.5; y' is 1 between 3.6 and 4.4; y is between 0.6 and 3.5; z is between 0.00 and 1.44; a designates that the V--Ni--Ti--Zr--Cr component as a group is from 70 to 100 atomic percent of the alloy, b,c,d,e, . . . , are the coefficients on the modifiers and M', M", M.sup.iii, and M.sup.iv are modifiers which may be individually or collectively up to 30 atomic percent of the total alloy The modifiers M', M", M.sup.iii, and M.sup.iv are chosen from Al, Mn, Mo, Cu, W, Fe, Co, and combinations thereof.
It is an object of the present invention to provide a new class of hydrogen storage compounds that can be used for various applications.
It is another object of the present invention to provide a hydrogen storage compound suitable for use in electrochemical cells, preferably rechargeable electrochemical cells.
It is another object of the invention to provide a hydrogen storage compound ideally suited for use in rechargeable nickel metal hydride cells.
These objects, together with other and further objects of the invention will become fully apparent from the following description.